As a current type light emitting device, OLED is increasingly applied to a high performance display. Conventional Passive Matrix OLED (PMOLED) requires shorter single pixel driving time for display, and thus needs increasing a transient current, rendering the increase of the power consumption; meanwhile, the employment of large current causes voltage drop of the Indium Tin Oxide (ITO) line to decrease too much, rendering the operation voltage of OLED too high and in turn the efficiency of OLED lower. An Active Matrix OLED (AMOLED) inputs OLED current via switching transistors by progressive scanning for display, which can solve the above problems very well.
Firstly, as an example, in the design for the backboard of AMOLED, a low temperature poly-Si Thin Film Transistor (LTPS TFT) is mostly adopted in AMOLED to constitute a pixel circuit for providing the corresponding current for the AMOLED device. As compared to the conventional amorphous-si TFT, LTPS TFT has a higher mobility and a more stable characteristics, and thus is more suitable to be used in an AMOLED display. However, due to the limitation of the crystallization process, LTPS TFTs, which are manufactured on a large glass substrate, have non-uniformity in electrical parameters such as threshold voltage, mobility, etc, and such non-uniformity may result in variances of current and luminance of OLED which can be perceived by human eyes, i.e., Mura phenomenon.
Secondly, in a large size display application, there is a certain resistance in the power cord of the backboard, and all of pixels are provide with driving current by the positive power supply (ARVDD) of the backboard, so the supply voltage in the area near the location of the power supply ARVDD is higher than that in the area located far from the location of the power supply ARVDD, and such phenomenon is called IR Drop. As the current of OLED depends on the voltage of ARVDD, IR Drop also results in variances of current in different areas, and Mura phenomenon in turn occurs in display.
Thirdly, there is also the non-uniformity in electrical parameters due to the non-evenness of the film thickness generated when OLED device is evaporated. FIG. 1 illustrates a schematic relationship between the luminance and the operation time of OLED and relationship between the threshold voltage and the operation time of OLED, wherein  denotes luminance of OLED and  denotes threshold voltage of OLED. As shown in FIG. 1, after OLED operates for a long time, the deterioration of the internal electrical performance of OLED results in the rise of the threshold voltage VOLED—0, and thus luminance efficiency decreases and luminance lowers.
It becomes an important issue that how to compensate the aging of the OLED device, since the aging of OLED causes Image Sticking to present in the area which displays a fixed picture for long time, affecting the display effect.
FIG. 2 illustrates a schematic relationship between luminance loss and threshold voltage of OLED, and FIG. 3 illustrates a schematic relationship between luminance and current density of OLED. As shown in FIG. 3, “” denotes the relationship between the luminance of red light OLED and the current density, “” denotes the relationship between the luminance of green light OLED and the current density, and “” denotes the relationship between the luminance of blue light OLED and the current density. As illustrated in FIG. 2 and FIG. 3, a substantially linear relationship is represented between the rise of threshold voltage and the luminance loss of OLED, and a linear relationship is also represented between the current density and the luminance of OLED. Therefore, when compensating the aging of OLED, the luminance loss can be compensated by increasing the driving current of OLED linearly as the threshold voltage of OLED increases.
AMOLED can be classed into three types in driving mode, i.e., digital driving mode, current driving mode, and voltage driving mode. The digital driving mode achieves a grey level by controlling driving time via TFT as a switch without compensating non-uniformity. Nevertheless, the operation frequency will be multiplied as the size of a display increases, which results in a high power consumption and to some extent reaches the physical limit of design. Therefore, the digital driving mode is not suitable for a large size display. The current driving mode achieves a grey level by providing different current to the driving transistors directly, which can compensate the non-uniformity of TFT and IR Drop. However, when a signal of a low grey level is written, the time for writing is prolonged too much since it is a small current to charge the large parasitic capacitance on a data line. Such a problem is more serious in a large size display and is difficult to be overcome. The voltage driving mode is similar to the conventional AMLCD driving mode, wherein a voltage signal representing a grey level is provided by a driving IC, and the voltage signal is converted to a current signal of a driving transistor inside a pixel circuit, and then the current signal is used to drive OLED to achieve luminance grey level. The voltage driving mode has such advantages as high driving speed and simplicity of implementation, and thus is suitable for driving a large size panel and is widely used in the art. However, extra devices such as TFTs and capacitors to compensate non-uniformity of TFT and IR Drop will be required.
FIG. 4 is a schematic diagram showing structure of a conventional pixel unit circuit of voltage driving type which comprises 2 TFT transistors, 1 capacitor and an OLED. A switching transistor T2 transmits a data voltage from a data line to a gate of a driving transistor T1, the driving transistor T1 converts the data voltage to a corresponding current and supplies the same to the OLED. In normal operation, the driving transistor T1 should operate in a saturation area, and should provide a constant current during a scanning time for one line. The current can be expressed as follows:
      I    OLED    =            1      2        ⁢                  μ        P            ·              C        ox            ·              W        L            ·                        (                                    V              Data                        -            ARVDD            -                          V              th                                )                2            
wherein μP is the carrier mobility, COX is the capacitance of the oxide layer of the gate, W/L is the width/length ratio of the transistor, VData is the data voltage, ARVDD is the power supply voltage of AMOLED backboard which is shared by all pixel units, and Vth is the threshold voltage of the driving transistor. It can be known that if the threshold voltages Vth are different from one pixel unit to another, then there are variances between the currents. Moreover, even if a constant current is provided to an OLED device, the emitting luminance of OLED decreases as the aging of the OLED device.
At present, there are a variety of structures of pixel unit for compensating the non-uniformity of Vth and IR drop. However, some structures of pixel unit can compensate the non-uniformity of Vth of the driving transistor, but can not compensate IR Drop and the luminance loss due to the aging of OLED; some structures of pixel unit can compensate the non-uniformity of Vth of the driving transistor and IR Drop, but can not compensate the luminance loss due to the aging of OLED; some structures of pixel unit can compensate the non-uniformity of Vth of the driving transistor, IR Drop and the affect of the aging of OLED, but are not applicable to a large size panel since their structures belong to the current driving type; and some structures of pixel unit can compensate the affect of the aging of OLED, but can not compensate the non-uniformity of Vth and IR Drop. Therefore, it is impossible for the pixel circuit presented in the prior art to effectively compensate the non-uniformity of the threshold voltage Vth of TFT driving transistor, IR Drop of power supply voltage of backboard and the affect of the aging of OLED while applicable to a large size panel.